Google Scholar - Google Scholar is a freely accessible web search engine that indexes the full text of scholarly...
EasyBib - Automatic works cited and bibliography formatting for MLA, APA and Chicago/Turabian citation styles.
PubMed.gov - PubMed comprises more than 29 million citations for biomedical literature from MEDLINE, life science journals, and online books. Citations may include links to full-text content from PubMed Central and publisher web sites.
Paperpile - Clean and simple and reference management for the web. Sync your PDFs to Google Drive and cite your papers in Google Docs.
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Scinapse - Scinapse is a free, nonprofit, Academic search engine for papers, serviced by Pluto Network.